1. Field of the Invention
The present invention relates to a process for producing an aluminum plate for use as the support of a lithographic printing plate. Specifically, the present invention relates to a method of roughening an aluminum plate for use as the support for a lithographic plate. In particular, the present invention relates to a method suitable for roughening an aluminum plate which is prone to the appearance of streaks or grainy unevenness in surface quality due to directional differences in crystal grains, both defects being highly likely to occur in the conventional chemical etching techniques.
2. Description of the Related Art
A method commonly adopted for producing aluminum supports for lithographic printing plates is AC etching which employs either an ordinary sinusoidal wave current or a current of a special alternating waveform such as a rectangular wave. By feeding an alternating current with a suitable counter electrode usually made of graphite, the aluminum plate is subjected to roughening, typically in one cycle. The pits produced by this treatment are generally shallow and do not assure a long press life. Under the circumstances, various methods have been proposed with a view to producing aluminum plates that have a grained surface characterized by a uniform and dense distribution of pits having a relatively great depth compared to diameter and which, hence, are suitable for use as the support of printing plates. Examples include roughening with a current from a special electrolytic power supply (Unexamined Published Japanese Patent Application (kokai) No. 67507/1978), adjusting the ratio between the amount of electricity applied in the anode cycle of AC electrolytic roughening and that in the cathode cycle (Unexamined Published Japanese Patent Application (kokai) No. 65607/1979), using a current of a special waveform from a power supply (Unexamined Published Japanese Patent Application (kokai) No. 25381/1981) and a combination of different quantities of current supply per unit area (U.S. Pat. No. 4,272,342 which corresponds to Unexamined Published Japanese Patent Application (kokai) No. 29699/1981).
Conventionally, aluminum supports are produced by the following process: an aluminum ingot is melted and held at a specified temperature to make a slab (400 to 600 mm thick by 1,000 to 2,000 mm wide by 2,000 to 6,000 mm long); an impurity-containing structure of the slab""s surface is cut off in thickness of 3 to 10 mm by means of a facing machine; in order to remove the stress within the slab and homogenize its structure, the slab is heat treated by being held in a soaking furnace at 480 to 540xc2x0 C. for 6 to 12 h; the slab is subsequently annealed to provide a homogeneous rolled structure and finally cold rolled to a specified thickness so that it is straightened to give a high degree of flatness. The thus produced aluminum web has been used as the support of lithographic printing plates.
Aluminum plates can be produced by a process that eliminates the steps of intermediate annealing and soaking treatments from the conventional process of manufacturing aluminum plates. Using such aluminum plates or general-purpose aluminum plates as the support for lithographic printing plates is desirable from the viewpoints of saving energy and effective utilization of resources.
In fact, however, xe2x80x9cstreaksxe2x80x9d and grainy unevenness in surface quality have often appeared when the support lithographic printing plates are manufactured from the above-mentioned aluminum plates. This is usually explained as follows: in the progress of a chemical dissolving reaction in aluminum, the rate of dissolution varies with crystal direction, so the electrochemical pitting reaction of aluminum will proceed at varying rates with the crystal direction.
In short, the asperities produced by the difference in the rate of chemical dissolving reaction are visually seen as streaks or grainy unevenness in surface quality or, alternatively, the difference in pitting reaction depending on the crystal direction (e.g. the difference in the number or size of pits) is seen as streaks or grainy unevenness in surface quality.
The present invention provide a process for producing an aluminum plate for use as the support for lithographic printing plates that are free from defects called xe2x80x9cstreaksxe2x80x9d or xe2x80x9cgrainy unevenness in surface qualityxe2x80x9d.
The present invention provides a process for producing an aluminum support for lithographic printing plates that comprises preliminarily imparting fine asperities to an aluminum plate and thereafter roughening the plate electrochemically in an acidic aqueous solution.
The fine asperities imparted to an aluminum plate cause random reflection that makes streaks or grainy unevenness in surface quality difficult to be seen. The fine asperities also offer the advantage that honeycomb pits having an average diameter of 0.1 to 3 xcexcm can be generated uniformly irrespective or crystal direction in an aqueous solution of nitric acid to be subsequently formed.
The present invention provides a process for producing an aluminum support for lithographic printing plates that comprises in sequence the steps of:
(1) subjecting an aluminum plate to preliminary AC electrochemical roughening in an aqueous solution based on hydrochloric acid using electricity in an amount of 1 to 300 C/dm2;
(2) immersing the aluminum plate in an aqueous solution of an acid or an alkali to perform chemical etching or electropolishing with the aluminum plate being used as anode, so that the aluminum plate is dissolved in an amount of 0.01 to 1.5 g/m2;
(3) subjecting the aluminum plate to DC or AC electrochemical roughening in an aqueous solution based on nitric acid; and
(4) immersing the aluminum plate in an aqueous solution of an acid or an alkali to perform chemical etching or electropolishing with the aluminum plate being used as anode, so that the aluminum plate is dissolved in an amount of 0.01 to 10 g/m2.
In the present invention, the fine asperities imparted to an aluminum plate cause random reflection that makes streaks or grainy unevenness in surface quality difficult to be seen. The fine asperities also offer the advantage that honeycomb pits having an average diameter of 0.1 to 3 xcexcm can be generated uniformly irrespective or crystal direction in an aqueous solution of nitric acid to be subsequently formed.
Another advantage that can be obtained is that uniform honeycomb pits can be formed even in aluminum plates that contain fairly large amounts of trace elements and with which uniform electrochemical roughening is difficult to achieve in aqueous nitric acid.
The fine asperities imparted to an aluminum plate cause random reflection that makes streaks or grainy unevenness in surface quality difficult to be seen. The fine asperities also offer the advantage that honeycomb pits having an average diameter of 0, 1 to 3 xcexcm can be generated uniformly irrespective of crystal direction in an aqueous solution of nitric acid to be subsequently formed.
Thus, the present invention relates basically to a process for producing an aluminum support for producing lithographic printing plates that comprises in sequence the steps of:
(1) subjecting the aluminum plate to AC electrochemical roughening in an aqueous solution based on nitric acid at a temperature of 60 to 95xc2x0 C. so as to form honeycomb pits with an average diameter of 0.1 to 0.5 xcexcm using electricity in an amount of 1 to 150 C/dm2; and
(2) immersing the aluminum plate in an aqueous solution of an acid or an alkali to perform chemical etching or electropolishing with the aluminum plate being used as anode, so that the aluminum plate is dissolved in an amount of 0.01 to 10 g/m2.